The present invention relates to a system for adjusting an auxiliary, or, extension, screed of a paving machine with respect to a main screed.
In conventional asphalt paving operations, a self-propelled vehicle, known as a “tractor” is used having a hopper on the front end thereof. This hopper receives asphalt paving material, typically from a dump truck, and the tractor generally engages and pushes the truck forwardly as the truck empties its contents into the hopper.
The asphalt material is transferred from the hopper to the roadbed or other surface being paved, and the asphalt empties onto the roadbed in front of transversely extending screw augers. These augers transport the asphalt material laterally in front of an elongated plate, or “screed”, which compresses and compacts the asphalt downwardly to form a “mat” of paving material, ideally of uniform thickness and surface finish.
The screed is typically pulled behind the tractor and may move upwardly or downwardly with respect to the tractor, such screed being connected to the tractor by tow arms, or bars. The tow bars are pivotally connected to the tractor and pivot about an axis, or “tow points.” This arrangement effectively allows the screed to “float” with respect to the tractor as the screed is towed behind the tractor.
In order to control the thickness of the asphalt mat being formed by the asphalt screed, the height of the screed is generally varied by positioning the tow bars at selected elevations. The angle of attack of the screed must also be controlled to achieve the desired asphalt mat thickness and surface finish, and this is typically done by means of a crank at each end of the screed, rotation of the cranks causing corresponding height adjustments on each end of the screed.
A conventional screed is of a set width. However, in certain paving applications, particularly in applications such as driveways, parking lots, and the like, where varying asphalt mat widths are required, an adjustable, or extendable, screed arrangement is desirable. Extendable screeds have become common in the asphalt paving industry to achieve varying widths of a paved surface without interruption of the paving process. Typically, extendable screeds consist of a main screed section of fixed width and hydraulically extendable auxiliary screed sections capable of extending from each end of the main screed unit, such auxiliary sections generally being referred to as “extensions” or “extension screeds.”
In the normal operation of an asphalt paver, an operator makes adjustments in the attack angle of the screed to affect the depth of the asphalt mat being laid. This is achieved by raising or lowering the tow point on the tractor with a hydraulic cylinder unit, or more commonly in smaller paving machines, with the rotatable cranks. The cranks cause the main screed to pivot relative to the tow arm attached to the tractor, to thereby change the angle of attack of the main screed with respect to the direction of travel. With each of these adjustments of the main screed, the corresponding position of the extension screed will change due to the fact that it is mounted to the main screed. In other words, the extension screed is slaved to the movement of the main screed.
When the asphalt mat thickness changes, and the main screed is adjusted to float at a new paving depth, the main screed and extension screed will move about an arc with respect to the axis of rotation of the tow bars and with respect to the tractor. Consequently, this rotation of the screed about the tow point axis induces a change in the extension screed elevation with respect to the main screed elevation, since the extension screed, which either leads or trails the main screed, rotates in an arc having a different radial distance from the axis of rotation, than does the main screed. For example, the extension screed, if it is positioned in front of the main screed, moves through an arc having a shorter radial distance with respect to the main screed, or, in the event the extension screed follows behind the main screed, the extension screed would rotate through an arc having a longer radial distance compared to the main screed. In either case, the screed extension rotates to an elevation which is different relative to the main screed, and this creates a discontinuity, or “step”, between the asphalt mat formed by the main screed and the asphalt mat formed by the extension screed. This step is undesirable in that it results in an overall asphalt mat having a height difference in the surface thereof.
The extension screeds are preferably adjusted such that the asphalt mat surface produced by such screeds is matched to the surface of the main screed, and there is no step or discontinuity between the mat heights. The physical characteristics of the screeds being used and the depth of the asphalt mat being formed ordinarily influence the frequency and the degree to which the extension screeds require adjustment. Further, other factors contribute to the extension screed adjustment, such as the weight of the screed mechanism towed by the tractor, the length of the main and extension screed plates itself from front to rear, and the relative location of the trailing edge-of the extension screed plate in relation to the trailing edge of the main screed plate.
In order to avoid a step or discontinuity in the mat, operators generally attempt to adjust the height of the extension screed relative to the main screed. This may result in a trial and error approach which can be time consuming and inefficient.
When smaller paving machines are used, such as the type ordinarily used to form residential driveways, parking areas, and the like, the length of the tow arms is shorter than on larger paving machines. The relatively short length of the tow arms of the small utility paver thus make the effect of depth differentials between the main screed mat and the extension screed mat more pronounced and dramatic, and the requisite skill in adjusting for such mat height differentials more acute. Given the general utility nature of such paving machines and the variety of applications which they encounter, frequent depth adjustment of the mat being formed is common. Thus, the operator of the paver must constantly closely monitor the paving operation, and often times the matching of the mat heights for the main and extension screeds is performed inadequately.
Devices for adjusting the extension screeds for paving machines have been patented. For example, U.S. Pat. No. 4,379,653, issued to Brown; U.S. Pat. No. 4,702,642, issued to Musil; U.S. Pat. No. 6,203,243B1, issued to Birtchet; and U.S. Pat. No. 5,222,829, issued to Mogler, et al., disclose screed adjustment arrangements.
Accordingly, there exists a need for a reliable, and easy to use screed adjustment system for use on paving machines.